Electrodeposition of polymeric films from aqueous dispersions onto conductive surfaces is an attractive method for producing adherent films of uniform thickness. My U.S. Pat. No. 4,592,816 describes such a process for electrodepositing a photosensitive film which can be used as a aqueously developable, negative acting photoresist to transfer an image pattern onto a conductive surface such as a printed circuit board. The photosensitive composition used in my prior invention is an aqueous dispersion containing at least one polymer free of ethylenic unsaturation and having charged carrier groups, a photoinitiator and an unsaturated crosslinking monomer. The electrodeposited photosensitive film can be selectively exposed to a source of actinic radiation to create a crosslinked polymer image on the surface corresponding to the image pattern described on a photomask, while the unexposed portion can be removed from the surface using an aqueous developer solution. Any defect, however small, such as for example a pinhole, in the imaged film can leave the conductive surface exposed and result in an incomplete circuit.
Electrodeposition of polymers (electrophoretic coating) generally produces good quality films of uniform thickness. The self limiting nature of the process assures that all conductive surfaces are coated with an even thickness of the film. In principal, this process produces films free of pinholes. A pinhole in the film leaves the conductive surface of the substrate exposed, allowing the passage of current which would initiate the deposition of material in the pinhole. In practice, however, pinholes are observed in films applied by electrodeposition.
The coalescence of the emulsion particles into a film on the conductive surface depends on hydroxide ions produced at the cathode surface by the electrolysis of water. Hydrogen evolution is also a product of the electrolysis of water occurring during the electrodeposition and can lead to pinholes in the film deposited by cationic electrodeposition (cataphoretic coating).
The emulsion particles in most emulsions for cationic electrodeposition are stabilized by the presence of protonated amines on the surface of the particle. These form a hydrophilic shell around the hydrophobic interior of the stabilized particles in the aqueous matrix. The low pH of the bath keeps the amines protonated. During the electrodeposition, these charged particles migrate to the surface of the cathode under the influence of the potential applied across the electrodes. The pH at the cathode surface is much higher than the pH of the bulk of the emulsion due to the hydroxide ions produced during the electrolysis of the water. The protonated amines are deprotonated and the particle becomes hydrophobic. The emulsion particle is no longer stable in the aqueous medium because the hydrophilic shell of protonated amines has been destroyed. These deprotonated particles coalesce to form a film on the surface of the cathode.
The hydrogen, a product of the electrolysis of water, is necessary to implement the electrodeposition of the polymer. Eliminating the electrolysis of water would eliminate the hydrogen bubbles which produce pinholes, but the polymer film would not be electrodeposited since no hydroxide would be produced.
The hydrogen gas produced at the cathode can produce defects at various stages of the film formation. A hydrogen bubble formed on the surface of the substrate before the film has begun to deposit displaces the emulsion and prevents deposition in the area covered by the bubble. This results in a pinhole in the final film. A bubble can also form under the film after the film deposition has started. This bubble can grow, lifting the film from the surface of the substrate (cathode), forming a void under the thin layer film. No further deposition takes place since the bubble has excluded the emulsion and acts as an electrical insulator. This can occur before the film has reached its full thickness. The final film suffers voids covered with only a thin film of the polymeric material. These defects are unsightly and reduce the protection provided electrodeposited coating. The thin film over the void can also break on drying or baking to produce a pinhole.
The problem of pinholes is usually addressed by formulating compositions used to form a film which lead to few pinholes. However, few principles are known for the formulation of compositions producing few pinholes, and trial and error is usually used. The deposition conditions can also be optimized to produce the fewest pinholes. Pinholes in the film can be eliminated by baking the film. This softens the film so that it flows into the pinholes.
Japanese Patent 56069836-A describes the use of unsaturated compounds to absorb hydrogen gas during the electrodeposition of a glass dispersion onto a silicon surface to prevent pinholes in coating. This Japanese patent describes only the use of unsaturated compounds with electron withdrawing groups attached to the alpha position, such as acrylic acid and its esters, methacrylic acid and its esters, acrylonitrile, and styrene. The electrodeposition was carried out at high voltages (450 V). No example of depositing an organic film is given. The compounds described in this Japanese patent showed little or no utility for eliminating hydrogen evolution in the deposition of organic films from emulsions.
The reduction of compounds with hydrogen in the presence of a catalyst, known as catalytic hydrogenation, is well known and widely used. Catalytic hydrogenation is usually carried out by sealing hydrogen gas, the compound to be reduced and the catalyst in a vessel. The reaction mixture is agitated until the reaction is complete. Heat and pressure are frequently needed to complete the reaction in a reasonable amount of time. Noble metals (and compounds of noble metals) are generally the most effective catalysts although base metals are also used. The catalyst is necessary because the addition of the hydrogen to the compound does not take place directly. The hydrogen is first adsorbed onto the surface of the catalyst, and it is this adsorbed hydrogen that reduces the compound.
A similar technique for reducing compounds that has received much less attention is electrocatalytic hydrogenation (electrohydrogenation). Usually, the catalyst in this technique also acts as a cathode in an electrolytic cell. In some cases the catalyst is coated onto a cathode which is not catalytic. The hydrogen produced by the electrolysis is adsorbed onto the catalyst surface. Reducible compounds dissolved in the electrolyte are then reduced by the hydrogen adsorbed on the surface of the cathode/catalyst. The objective of electrocatalytic hydrogenation is to prepare the reduced product. The process of the present invention uses a similar technique but the objective is to consume the hydrogen before it forms gas bubbles which cause defects in the film during electrodeposition.
It is the object of the present invention to eliminate or reduce the pinhole defects in cataphoretically deposited films. It is a further object of the present invention to incorporate a compound into a cataphoretically deposited photosensitive composition to decrease hydrogen gas evolution without interfering with the cataphoretically deposited films.